I really like gravit.io as an online lightweight version of Illustrator. For me it has been especially good for online targeted diagrams, e.g., for blog posts & web pages.
For my shameless plug, I helped build a whiteboarding tool that is great for simple hand-drawn diagrams, and truly looks whiteboardy. https://limnu.com. We intentionally kept the tools simple so you can iterate and obsess over the idea rather than the look. That might not be what you mean by “beautiful diagrams”, but personally I find a good whiteboard diagram quite beautiful. Maybe it’s a functional beauty. We used Limnu to create all the diagrams for the online book “Ray Tracing in One Weekend.” http://www.realtimerendering.com/raytracing/Ray%20Tracing%20...
I also worked at LucidChart for a while, and I know first-hand it’s very powerful and quite good compared to almost everything online, especially for flowcharts and serveral pre-baked styles of data driven diagrams.
Recently I used plotdevice.io to build a 3d line renderer for a bunch of diagrams in a book. It’s a Mac environment that is similar to Processing, but is based on Python and has built-in PDF export.